1. Field of the Invention
The present invention relates to fume exhaust duct connections for fiberglass reinforced plastic duct sections, and more particularly to a joint including a collar that provides an improved method for joining duct sections after applying a sealant to form a circumferential joint bond between pairs of adjoining duct section ends.
2. Description of the Related Art
Ductwork for corrosive vapor exhaust systems is used extensively in many diverse industries which utilize hazardous chemicals to process raw materials or perform manufacturing procedures, such as the semiconductor, plating, and pharmaceutical industries. Such ductwork also is required in the many research and development laboratories which use highly reactive, toxic or otherwise hazardous chemicals. Such chemicals not only can put workers at risk to hazardous fumes, but also are potential sources of contamination of industrial processes or laboratory experiments. Consequently, vapors from hazardous chemicals must be exhausted through leak-proof air ducts to safely remove them from work areas. Duct installations can be very large, consisting of many thousands of feet of ductwork which may be manifolded and connected to multiple exhaust fans. Because of the wide diversity of chemicals used in industrial and research applications, it is extremely difficult to provide a single material for fabricating ductwork which can withstand all the chemicals to which duct interiors may be exposed. Over the past forty years the trend in materials has been away from bare and coated metals and toward the use of plastics, particularly fiberglass reinforced plastics (FRP's) which incorporate various types of resins.
In addition to the problem of providing a duct capable of resisting broad classes of chemical vapors, there is also the problem of providing adequate resistance to fire. Unlike metallic ducts, plastic ducts exhausting chemicals which can react exothermically or with the duct interior surface are at risk to being set on fire. A problem common to all plastics has been flammability. Plastics can burn rapidly and produce toxic smoke, creating hazards of their own.
U.S. Pat. No. 5,298,299 to L. E. Shea, entitled "Double Wall Fireproof Duct", which is incorporated in its entirety herein by reference, is directed to a dual-laminate tubular duct section having both good chemical resistance and good fire resistance. An inner laminate is formed by coating a mylar wrapped mandrel with a chemically resistant resin and then wrapping the mandrel with successive layers of FRP fabric saturated with the resin. An outer laminate is then formed directly over the inner laminate by applying successive layers of FRP fabric saturated with a fire-retardant resin.
U.S. Pat. No. 5,549,949 ("'949") to D. Williams et al., entitled "Fume Duct Circumferential Joint Sealant", which is incorporated in its entirety herein by reference, is directed to sealant compositions for sealing the circumferential joint between pairs of dual-laminate fume duct sections, and to a joint sealing method which provides strong bonding between the sealant and laminate surfaces which typically are phenolic/glass and vinyl ester. As discussed therein, ducts are fabricated as sections of standard length(s) which are transported to a job site and assembled there. Since a leak-proof joint is required between each pair of contiguous sections, even the smallest installation requires a considerable number of such joints. Joints must not only prevent fumes from escaping in day-to-day operation, but must also remain leak-proof after prolonged exposure to corrosive or otherwise reactive chemicals. Also, joints must not fail catastrophically in the event a flame propagates through the interior or, if exposed directly to heat such as from a fire external to the ductwork, fail mechanically or become a source of smoke particulates and other contaminants. Because mechanical interfacing cannot by itself prevent leakage, a sealant must be applied circumferentially to each interface. The '949 patent provides a solution to simplifying what had been the most time-consuming step in joining dual-laminate sections, viz., preparing the resin-impregnated surfaces to which the sealant must bond in order to effect a leak-proof seal. Unless mating surfaces near the ends of each duct section were first sanded or otherwise polished, the interposing sealant layer would not uniformly adhere to the surfaces, resulting in porosities in the hardened sealant through which fumes could leak. These surfaces include the opposed end portions of the inner laminate surface, the opposed end portions of the outer laminate surface, and the exterior surfaces of a "slip" collar interposed internally between each pair of end sections.
By enabling the time and labor needed to assemble multi-section ductwork to be reduced, use of the sealant disclosed in the '949 patent can be an important factor in improving the profitability of businesses which install fume ducts. Many installations, however, require joints to have greater integrity to rupture from tensile and flexural loading than can be provided using only mechanical interfacing between the duct sections and slip collar, and sealant bonding. For example, ductwork disposed exterior to a building must be able to withstand flexure due to wind shear. Also, high velocity gases resulting from an explosive chemical reaction within a localized portion of a duct can create a large overpressure, stressing nearby joints even if the duct sections remain intact. The method disclosed in the '949 patent for reinforcing a joint is to form a "lay-up" bond by tightly winding alternate layers of fine boat cloth mesh and a combination of fiberglass sheeting and coarse woven roving mesh around the joint seam. The larger the duct diameter, the more layers must be used. Each time a dry layer is wound, it must be "wet out" with a resin component of the sealant. Lay-up reinforcement substantially strengthens the joint to the extent that under tensile loading the duct material is likely to rupture before the joint fails. While not as significant a cost driver as sanding, the lay-up method also entails considerable time and labor. Consequently, there is a need for a faster, easier and thereby more cost-effective method for maintaining duct section joint integrity against internal and external tensile and flexural loading. To streamline ductwork assembly, attachment of a device implementing the method should be simple and reliable.
A device for connecting together two FRP duct sections, each having a terminal end portion of equal diameter, is disclosed in U.S. Pat. No. 5,505,497 ("'497") to L.E. Shea et al., entitled "Mechanical Joint Connections for Fiberglass Reinforced Duct Sections". The device includes two end gaskets each covering and sealed to an end portion, a central gasket covering the two end gaskets and extending around both end portions, two spacer gaskets supporting a flexible metal sleeve surrounding the central gasket, and tightening devices on the sleeve which compress the central gasket and hold the aligned duct sections together. The tightening devices are spring-driven so that if the elastomeric gaskets are reduced due to fire, the sleeve will maintain a firm non-slip hold on the duct joint. The '497 device is not adapted for use with joints that incorporate an internal slip collar, and makes minimal use of sealant because FRP surfaces mate with gaskets rather than with other FRP surfaces. The sleeve acts to compress the gaskets and keep the duct sections aligned, but does not enhance structural integrity against tensile and flexural loads.
Joints assembled according to the lay-up method are potentially susceptible to failure due to heating from flames interior or exterior to the ductwork. Even if a joint interior is not heated to a temperature high enough to cause the duct material to melt or burn through, the sealant can melt, bum or otherwise decompose to an extent that gases leak out. Similarly, if the joint exterior is heated, the sealant coating exterior mating surfaces can melt. At sufficiently high temperature, the lay-up wrapping can begin to burn, greatly reducing structural integrity and making the joint vulnerable to rupture from a normal flexural load.
Fire retardant materials that have the ability to remain in an unexpanded elastomeric form until heat from a fire causes "intumescent" expansion to fill burned-out voids with a fire-resistant barrier are widely used in gaskets and as caulking. For example, the central gasket of the '497 device may contain an intumescent material which causes it to swell up and increase in volume in response to elevated temperatures caused by fire within the duct sections. Alternatively, the central gasket may include a flexible bag enclosing a flowable quantity of intumescent material held tightly by the sleeve. The bag would swell and increase in volume if the-duct sections are partially consumed by fire, thereby serving to maintain the structural integrity of the joint.